https://github.com/00-Evan/shattered-pixel-dungeon/blob/2bb34a4e91d29c8785a9363cad6ddfe5122b1d4f/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/items/potions/Potion.java#L515 With 3 different seeds, there is a 50% chance of a fully random potion. With 2 different seeds, there is a 25% chance of a fully random potion.

https://github.com/00-Evan/shattered-pixel-dungeon/blob/2bb34a4e91d29c8785a9363cad6ddfe5122b1d4f/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/items/potions/Potion.java#L538 The 1st healing potion is garunteed and the limit is 9, every brew decreasing the chance.

oh thanks. it still mean that I got 4-5 times the "1/4 fully random" :) I had a lot of healing potions dropped by bats and all, but it was my first alchemy healing. and 3 green seeds should have given me a health.. :/

The chance of producing a potion of healing decreases with each succesfully crafted one. You can craft a random one instead even after crafting just one.